Currently, 3GPP is considering development of E-UTRA and E-UTRAN as set out in the technical specification 3GPP TS 36.300 v 8.4.0 (March 2008), incorporated herein by way of reference, and related documents. 3GPP Long Term Evolution (LTE) aims to enhance the Universal Mobile Telecommunications System (UMTS) standard, for example, by improving efficiency and services.
In E-UTRAN, user equipment (UE) communicates with a network node, NodeB (eNB), with data being sent on radio bearers (RBs) over a radio link between them. The eNB interfaces with a Mobile Management Entity (MME) via an interface designated as S1. The E-UTRAN network includes a plurality of eNBs and MMEs and the UE may be handed over from one eNB to another to achieve optimum performance, for example under fading channel conditions or where the UE is moving from one cell coverage area to another. Neighboring eNBs may communicate with one another over the X2 interface between them.
The connection between the UE and the source eNB to which it is attached may be lost, this being known as radio link failure (RLF). It had previously been proposed that a UE experiencing an RLF and reappearing in a cell of another eNB would be handled as a transition via an idle state, RRC_IDLE. In this mechanism, when there is RLF in the source eNB, the UE enters IDLE state and selects a new target eNB. When the UE has attached to the new target eNB, and received the system information for the target eNB, the UE may perform the access procedure to establish resources in the target eNB.
RLF may be particularly likely during fading channel conditions, when handover from the source eNB to a target eNB may be imminent. To avoid going via the IDLE state, it has been suggested that a new eNB could be prepared beforehand to accept the UE by using the handover preparation procedure. The Handover_Request message can be sent to multiple eNBs which are thus able to recognize the UE, each of the eNBs being prepared as handover candidates. Then, the UE is able to carry on with its old context after having chosen the new target eNB during the mobility phase of the RLF.
In advance of LTE handover, the UE performs measurements and applies handover parameters and equations for handover event creation as specified in the 3GPP RRC protocol specification 3GPP TS 36.300 v 8.4.0 and as exemplarily shown in FIG. 1, which illustrates signal strength variation with time and the application of Handover Margin (HOM), Cell Individual Offset (OCN) and Time to Trigger Handover (TTTH) to generate creation of a handover event. Parameters other than those illustrated may be used depending on the particular network implementation.
As soon as a handover event has been generated, the UE includes this event into a MeasurementReport message and forwards it to the source eNB. The source eNB decides for or against handover. Where handover is selected, the source eNB sends a HandoverRequest message via the X2 interface to the target eNB and waits for the X2 HandoverRequestAcknowledge message, which includes a Transparent Container, from the target eNB.
After acknowledgment, the source eNB inserts the Transparent Container into the HandoverCommand (RRCHandoverReconfiguration) message and forwards it to the UE. With this message, the UE is commanded to perform handover.
The UE synchronizes to the target eNB and sends the HandoverConfirm (RRCReconfigurationComplete) message to the target eNB to indicate successful attachment. After path switch, the target eNB delivers the UEContextRelease message via the X2 interface to the source eNB.
A successful handover procedure is one of the main key performance indicators for a mobile system. Failures during or in relation to the handover procedure are thus undesirable. In LTE, three kinds of failures might occur during or in relation to the handover procedure: Radio Link Failure (RLF) prior to handover; Handover command failure (leading to RLF); or Access failure (leading to Handover failure). Currently, Self-Organizing Network (SON) functionality is being considered for LTE. One possibility where SON functionality might be applied is in Handover Optimization in an effort to reduce failures. This may involve, for example, adapting handover parameters, such as, for example, Time to Trigger Handover (TTTH), Handover Margin (HOM), Filtering coefficient and Cell Individual Offset (OCN) for future handover event generation. Also, the Modulation and Coding Scheme (MCS) and transmission power may be adapted for subsequent HandoverCommand transmission cases. Another approach, depending on the cause of failure, might be to adjust the decision parameters for preparation of multiple eNBs.
At present, it is proposed that in any of the three failure cases mentioned above, that is, RLF prior to handover, Handover command failure and Access failure, the UE should enter a second phase of the Radio Link Failure procedure. In this mechanism, there is a UE-controlled attachment at a suitable “best available” cell, which may be a cell of the source eNB, a cell of the target eNB or a cell from another, new eNB. In the case when the UE attaches to a cell of the source eNB or the target eNB, the communication will be maintained. In the case the UE attaches to a new target eNB, the connection will break if the UE context is unknown.